The invention relates to a circuitry for measuring piezoelectric signals and more specifically to a circuitry which allows adjustment of the range. As such, it is known from Swiss Patent 542.434. Corresponding to U.S. Pat. No. 4,009,447. With this known circuitry, the range or feedback capacitance is fully discharged when resetting, so that before a renewed switch-on of the power for a fresh measurement, there is no voltage on this capacitance and this process is consequently called zeroing. The disadvantage of this zeroing of the range capacitor is that measurements can be made only in one range determined by its capacitance, as will be explained later.
The purpose of the invention is to achieve a circuitry with a measuring range adjustable within certain limits, freed from direct linkage to the capacitance of the range capacitor. This aim is attained by the features characterized in of this invention.
By means of the voltage divider, after switching on the power, i.e. upon breaking the reset short circuit of the two-wire line, an output quiescent voltage, depending on the voltage divider selected, is built up to a level that determines the measuring range to be covered. This voltage build-up is produced by a pulse of defined duration closing the switching element for this defined time interval when the power is switched on. This causes a control loop to charge the range capacitor to a defined voltage. The reference voltage at one input of the amplifier serves at setpoint, while as actual value the output voltage over the voltage divider and the switching element at the other end of the amplifier is brought into line with the reference voltage. At the end of the defined time interval, the circuit is ready for measuring.
If the actual value is too big for the control system as just described, the output voltage must be lowered. The current passing during this is led off via the input components, causing these to heat up, leading to increased drift. To minimize this heating, a control element regulated by the amplifier, for example a variable transistor, may be shunted with the voltage divider which serves to determine the output quiescent voltage. Via this transistor the rest of the supply current can be led off past the input components.
The time constant of the circuit, and with it the lower cutoff frequency, may be adjusted within certain limits by selecting a suitable time constant resistor of Rg rating, allowing for example switchover between operation with relatively short time constant and a long operating state with a time constant as long as possible. In the short mode, with a given range capacitance Cg, the time constant is determined essentially by the rating Rg of the time constant resistor. In long mode, the resistance of the open switching element is determined essentially by the time constant. Switching on or over to one of the two operating modes can be effected with the new circuitry if the pulse generator controls an asymmetrical bistable trigger element in addition. The particular final state of the trigger element, which is determined by a direct, steep or a delayed, relatively flat rise of the switch-on pulse, defines the time constant of the circuit. Moreover a unidirection of element, such as a diode prevents self-control of the final state of the trigger element determined by the steep rise.
If a relatively long time constant, exceeding 1 second for example, is required for the circuit in short mode operation too, resulting in a relatively slow reset, i.e. relatively slow charging of the range capacitor to the output quiescent voltage, for speedy reset, a further switching element likewise controlled by the pulse generator may bridge the time constant resistor. The mode in which the circuit is being operated can be seen if the voltage divider determining the output quiescent voltage is preceded by a shortable resistor whose short circuiting is controlled by the trigger element. The output quiescent voltage is altered by switching-in or shorting this resistor.
A zero offset, due to a drift current in the time constant resistor, can be corrected by shunting this with an analog integrator or comparator. The drift current then no longer passes through the time constant resistor but is led off via the analog integrator or comparator.
For a given range capacitor, the measuring range can be optimized for the proposed application by feeding the range capacitor back through a second voltage divider. The measuring signal input of the amplifier is preceded by a low-pass filter which serves to compensate any frequency response change caused by adjusting the measuring range with the second voltage divider.
If the piezoelectric element of the piezotransducer is fitted in the transducer housing insulated against earth and placed between both amplifier inputs, the drift in the piezotransducer/ amplifier combination can be kept low, because the voltage on the piezoelectric element is low or zero.